[Feature request] Table of contents for Python API

[ivirshup]

Is your feature request related to a problem? Please describe.

It's a little difficult to navigate the docs and see what classes exist. At the moment it looks like I need to scroll through the docs.

Describe the solution you'd like

It would be great if there was a heading level which let me see a listing of classes in a module.

[pablo-gar]

@ebezzi please apply this change to the Census doc-site as well. I believe the desired functionality is something like scanpy that I fully support we implement